1992年05月TOEFL阅读真题

EXERCISE FOURTEEN

Passage 1

In 1781 twelve families trooped north from Mexico to California. On a stream along the desert's edge they built a settlement called Los Angeles. For many years it was a market town where nearby farmers and ranchers met to trade.

Then in 1876 a railroad linked Los Angeles to San Francisco and, through San Francisco to the rest of the country. The next year farmers sent their first trainload of oranges east. By a new railroad provided a direct route between Los Angeles and Chicago.

Then in the 1890 's oil was discovered in the city. As derricks went up, workers built many highways and pipe lines. Digging began on a harbor that would make Los Angeles not only an ocean port but also a fishing center. The harbor was completed in 1914. That year the Panama Canal opened. Suddenly Los Angeles was the busiest port on the Pacific Coast.

Today the city is the main industrial center in the West. It produces goods not only for other West Coast communities but also for those in other parts of the country. It leads the nation in making air planes and equipment for exploring outer space. Many motion pictures and television programs are filmed in Los Angeles. The city is also the business center for states in the West Improvements in transportation are the main reason for Los Angeles' growth

1.According to the passage what was the main commercial activity of Los Angeles during the

years directly following its settlemen?

(A) Fruit growing

(B) Oil drilling

(D) Trading

2.According to the passage in which year were oranges first shipped from Los angeles to the

East Coast by train?

(A) 1781

(B) 1876

(D) 1890

3.San Francisco is mentioned in the passage for which of the following reasons?

(A)The settlers who founded Los Angles came from San Francisco.

(B)San Francisco linked Los Angeles with the rest of the country

(C)San Francisco was a market town where farmers came to trade.

(D)Oil was discovered in San Francisco in the 1890's.

4.Where in the passage does the author state the principal cause of the expansion of los Angeles?

(A) Line 5

(B) Line 7

(I)) Lines 15-16

Passage 2

As many as one thousand years ago in the Southwest, the Hopi and Zuni tribes of North America were building with adobe-sun - baked brick plastered with mud. Their homes looked remarkably like modern apartment houses. Some were four stories high and contained quarters for perhaps a thousand people along with storerooms for grain and other goods. These buildings were usually put up against cliffs, both to make construction easier and for defense against enemies. They were really villages in themselves as later Spanish explores must have realized since they called them "pueblos", which is Spanish for town.

The people of the pueblos raised what are called "the three sisters"- corn, beans, and squash.

They made excellent pottery and wove marvelous baskets. some so fine that they could hold water. The Southwest has always been a dry country. with water scarce. The Hopi and Zuin brought water from streams to their fields and gardens through irrigation ditches. Water was so important that it played a major role in their religion. They developed elaborate ceremonies and religious rituals to bring rain.

The way of life of less settled groups was simpler and more strongly influenced by nature Small tribes such as the Shoshone and Ute wandered the dry and mountainous lands between the Rocky Mountains and the Pacific Ocean. They gathered seeds and hunted small animals such as rabbits and snakes. in the Far North the ancestors of toaa~ S inuit hunted seats.

walruses. and the great whales. They lived right or the frozen seas in shelters called igloos built of blocks of packed snow. When summer came they fished for salmon and hunted the lordly caribou.

The Cheyenne. Pawnee. and Sioux tribes. known as the Plains Indians. lived or the grasslands between the rocky Mountains and the Mississippi River. They hunted the bison, commonly called the buffalo. its meat was the chief food of these tribes. and its hide was used to make their clothing and the covering of their tents and tepees.

1.VJnicn of the following is the best title for the passage?.

(A) Building with Adobe

(B) One Thousand Years of Hunting

(C ) The Hopi and Zuni Tribes

(D)Early North American Societies

- 2.It can be interred from the passage that the dwellings of the Hopi and Zuni were

(A) very small

(B) family advanced

(D) extremely fragile

3.The author uses the phrase the three sisters in line 9 to refer to

(A )Hopi women

(B) family members

(D) rain ceremonies

4.Which of the following animals was most important to the Plains Indians?

(A) The salmon

(B) The caribou

(C)The seal

(D): The buffalo

5.According to the passage. which of the following tribes lived in the grasslands

(A)The Shoshone and Ute

(B)The Cheyenne and Sioux

(C)The Hop and Zuni

(D) The Pawnee and Inuit

6.Which of the following is NO mentioned by the author as a dwelling space of the first North Americans

(A)Log cabins

(B)Aaobe houses

(C)Tepees

(D)Igloos

7.The author groups North American Indians according to their

(A)tribes and environment

(B)impact on the Europeans

(C)rituals and ceremonies

(D)date of appearance on the continent

Passage 3

The term 'virus is derived from the Latin word for poison. or slime. It was originally applied to the noxious stench emanating from swamps that was thought to cause a variety of diseases in the centuries before microbes were discovered and specifically linked to illness.

But it was not until almost the end of the nineteenth century that a true virus was proven to be the cause of a disease.

The nature of viruses made them impossible to detect for many years even after bacteria had been discovered and studied. Not only are viruses too small to be seen with a light microscope. they also cannot be detected through their biological activity. except as it

occurs in conjunction with other organisms. In fact, viruses show no traces of biological activity by themselves. Unlike bacteria. they are not living agents in the strictest sense

Viruses are very simple pieces of organic material composed only of nucleic acid. either DNA or RNA. enclosed in a coat of protein made up of simple structural units. (Some viruses also contain carbohydrates and lipids.) They are parasites. requiring human. animal.

or plant cells to live. The virus replicates by attaching to a cell and injecting its nucleic acid.' once inside the cell. the DNA or RNA that contains the virus' genetic information takes over the cell's biological machinery. and the cell begins to manufacture viral proteins rather than its own.

1. Which of the following is the best title for the 4.

passage.

(A) New Developments in Viral Research

(B) Exploring the Causes of Disease

(D)understanding Viruses

2. Before microbes were discovered It was believed that some diseases were caused by

(A)germ - carrying insects

(B)certain strains of bacteria

(C)foul odors released from swamps

(D) slimy creatures living near swamps

3. The word "proven" in line 4 is closest meaning to which of the following.

(A)Shown

(B)Feared

C)Imagined

(D)Considered

4. The word nature" in line 6 is closest in meaning to which of the following?

(A)Self - sufficiency

(B)Shapes

(C)Characteristics

(D)Speed

5.The author implies that bacteria were investigated earlier than viruses because

(A) bacteria are easier to detect

(B) bacteria are harder to eradicate

(C)viruses are extremely poisonous

(D) viruses are found only in hot climates

6.All of the following may be components of a virus EXCEPT

(A) RNA

(B) plant cells

(D) a coat of protein

Passage 4

Dancer Martha Graham trained her body to move in different ways and in different

contexts from any before attempted. "life today is nervous, sharp, and zigzag," she said. "It often stops in midair. That is what I aim for in my dances." She insists she never started out to be a rebel. It was only that the ernotions she had to express could not be projected

through any of the traditional forms.

This was in 1925. All forms of art were undergoing a revolution. The theories of

psychology were being used to extend the boundaries of poetry, music, and painting.

Martha Graham's debut dance concert in her new idiom occurred on April 18, 1926.

Connoisseurs of dance, gathered at the Forty - eighth Street Theater in New York.

witnessed Martha Graham's first foray into this new realm of dance. They saw. through such dance sequences as "Three Gobi Maidens." and "A Study in Lacquer. desires and conflicts expressed through bodily movements. These critics agreed that something entirely new. a departure from all previous forms, had been witnessed.

In the early thirties. she founded the. Martha Graham School of Contemporary Dance.

Her classes were used as a laboratory for her stage works. and her stage works in turn were

a means for attaching new pupils to her school-a sort of self - winding process, with herself

as the key to the development.

Martha Graham and the school she has founded are virtually synonymous with the

modern dance. She had not only produced a technique of the dance. choreographed and

taught it. but her disciples have gone out to fill the modern dance world.

L. What does the passage mainly discuss.

(A)Martha Graham' S development of modern dance

(B) The revolution of art forms in the i920' s

(D)The Martha Graham School of Contemporary Dance

2.It can be inferred from the passage that. in the beginning of her career. Martha Graham

S mode of dance was

(A)readily accepted

(B) considered rebellious

(D)accepted only in New York

3. It can be inferred from the passage that Martha Graham' 5 style of dance differed from

traditional dance in the

(A type of movements

(B) speed of the performance

(C)variety of themes

(D)ages of the performers

4.In lines 16 – t 4.in lines 16-17,the author uses the phrase "a sort of self - winding

process" to illustrate

(A) the new steps Graham developed for dance

(B) the relationship between Graham' 5 performances and her school

(D)the physical endurance of Graham' 3 dancers

5.According to the passage. what is the present status of Martha Graham's work?

(A)It is historically interesting, but is no longer popular.

(B) It has evolved into something completely different.

(C)It is carried on by her students.

Passage 5

If the salinity of ocean waters is analyzed. it is found to vary only slightly from place to place. Nevertheless. some of these small changes are important. There are three basic

processes that cause a change in oceanic salinity. One of these 15 the subtraction of water from the ocean ~y means of evaporation-conversion' of liquid water to water vapor. In this manner. the salinity is increased. since the salts stay behind. If this is carried to the extreme.

of course. white crystals of salt would be left behind: this. by the way. is how much of the table salt we use is actually obtained.

The opposite of evaporation is precipitation. such as rain. by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation. or decreased by the addition of fresh water by precipitation or runoff.

Normally in tropical regions where the Sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much

evaporation. Similarly, in coastal regions where rivers dilute the sea salinity is somewhat lower than in other oceanic areas.

A third process by which salinity may be altered is associated with the formation and

melting of sea ice. When seawater is frozen, the dissolved materials are left behind. In this manner. seawater directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course. when this ice melts, it will tend to decrease the salinity of the surrounding water.

In the Weddell Sea, off Antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.

1. What does the passage mainly discuss?

(A)The bodies of water of the world

(B) The elements of salt

(D)The salinity of ocean water

2. According to the passage, the ocean generally has more salt in

(A) coastal areas

(B) tropical areas

(D) turbulent areas

3. All of the following are processes that decrease salinity EXCEPT

(A evaporation

(B)precipitation

(C)runoff

(D)melting

4. Which of the following statements about the salinity of a body of water can best be inferred from the passage?

(A) The temperature of the water is the most important factor.

(B) How quickly the water moves is directly related to the amount of alt.

(D) Various factors combine to cause variations in the salt content of water.

5. The word "it" in line 19 refers to which of the following?

(A) Sea ice

(B) Salinity

(D) Manner

6. Why does the author mention the Weddell Sea?

(A)To show that this body of water has salinity variations

(B)To compare Antarctic waters with Arctic waters

(C)To give an example of cold - water salinity

(D)To point out the location of deep waters

7.Which of the following is NOT a result of the formation of ocean ice?

(A)The salt remains in the water

(B)The surrounding water sinks

(C)Water salinity decreases

(D)The water becomes denser

8. What can be inferred about the water near the bottom of oceans?

(A)It is relatively warm.

(B)Its salinity is relatively high.

(C)It does not move.

(D)It is formed by melting sea ice.

老托福阅读真题及答案解析

老托福阅读真题及答案解析托福从听、说、读、写四方面进行英语能力全面考核。托福频道为大家提供了这四个方面的资料，希望对大家有所帮助。 Aviculturists, people who raise birds for commercial sale, have not yet learned how to simulate the natural incubation of parrot eggs in the wild. They continue to look for better ways to increase egg production and to improve chick survival rates. When parrots incubate their eggs in the wild, the temperature and humidity of the nest are controlled naturally. Heat is transferred from the bird's skin to the top portion of the eggshell, leaving the sides and bottom of the egg at a cooler temperature. This temperature gradient may be vital to successful hatching. Nest construction can contribute to this temperature gradient. Nests of loosely arranged sticks, rocks, or dirt are cooler in temperature at the bottom where the egg contacts the nesting material. Such nests also act as humidity regulators by allowing rain to drain into the bottom sections of the nest so that the eggs are not in direct contact with the water. As the water that collects in the bottom of the nest evaporates, the water vapor rises and is heated by the incubating bird, which adds significant humidity to the incubation environment. In artificial incubation programs, aviculturists remove eggs from the nests of parrots and incubate them under laboratory conditions. Most commercial incubators heat the eggs fairly evenly from top to bottom, thus ignoring the bird's method of natural incubation, and perhaps reducing the viability and survivability of the hatching chicks. When incubators are not used, aviculturists sometimes suspend wooden boxes outdoors to use as nests in which to place eggs. In areas where weather can become cold after eggs are laid, it is very important to maintain a deep foundation of nesting material to act as insulator against the cold bottom of the box. If eggs rest against the wooden bottom in extremely cold weather conditions, they can become chilled to a point where the embryo can no longer survive. Similarly, these boxes should be protected from direct sunlight to avoid high temperatures that are also fatal to the growing embryo. Nesting material should be added in sufficient amounts to avoid both extreme temperature situations mentioned above and assure that the eggs have a soft, secure place to rest. 1. What is the main idea of the passage ? (A) Nesting material varies according to the parrots' environment. (B) Humidity is an important factor in incubating parrots' eggs. (C) Aviculturists have constructed the ideal nest box for parrots. (D) Wild parrots' nests provide information useful for artificial incubation. 2. The word "They" in line 2 refers to

老托福阅读真题及答案：PASSAGE 19

老托福阅读真题及答案：PASSAGE 19 The principal difference between urban growth in Europe and in the North American colonies was the slow evolution of cities in the former and their rapid growth in the latter. In Europe they grew over a period of centuries from town economies to their present urban structure. In North America, they started as wilderness communities and developed to mature urbanism in little more than a century. In the early colonial days in North America, small cities sprang up along the Atlantic Coastline, mostly in what are now New England and Middle Atlantic states in the United States and in the lower Saint Lawrence valley in Canada. This was natural because these areas were nearest to England and France, particularly England, from which most capital goods (assets such as equipment) and many consumer goods were imported. Merchandising establishments were, accordingly, advantageously located in port cities from which goods could be readily distributed to interior settlements. Here, too, were the favored locations for processing raw materials prior to export. Boston, Philadelphia, New York, Montreal, and other cities flourished, and, as the colonies grew, these cities increased in importance. This was less true in the colonial South, where life centered around large farms, known as plantations, rather than around towns, as was the case in the areas further north along the Atlantic coastline. The local isolation and the economic self-sufficiency of the plantations were antagonistic to the development of the towns. The plantations maintained their independence because they were located on navigable streams and each had a wharf accessible to the small shipping of that day. In fact, one of the strongest factors in the selection of plantation land was the desire to have its front on a water highway. When the United States became an independent nation in 1776, it did not have a single city as large as 50,000 inhabitants, but by 1820 it had a city of more than 10,000 people, and by 1880 it had recorded a city of over one million. It was not until after 1823, after the mechanization of the spinning had weaving industries,

历年托福考试阅读真题汇总含答案

0308 托福试题阅读( 55minutes ) Question 1-11 seen in putrefying materials .He did (10) this by passing air through guncotton filters, the fibers of which stop solid particles. After the guncotton was dissolved in a mixture of alcohol and ether, the particles that it had trapped fell to the bottom of the liquid and were examined on a microscope slide .Pasteur found that in ordinary air these exists a variety of solid structures ranging in size from 0.01 mm to more than 1.0 mm .Many of these bodies resembled the reproductive (15) structures of common molds, single-celled animals, and various other microbial cells . As many as 20 to 30 of them were found in fifteen If food is allowed to stand for some time, putrefies .When the putrefied material is examined microscopically ,it is teeming with bacteria. Where do these bacteria come from , since they are fresh food? Even until the mid-nineteenth century, many people believed microorganisms originated by spontaneous (5 ) generation ,a hypothetical living organisms develop from nonliving matter. The most powerful spontaneous generation microbiologist Louis showed that structures present in air closely found not that it to be seen in such process by which of the theory of French chemist and opponent was the Pasteur(1822-1895).Pasteur resemble the microorganisms

2020年托福阅读模拟试题及答案(卷九)

2020年托福阅读模拟试题及答案（卷九）托福阅读文本： The Native American peoples of the north Pacific Coast created a highly complex maritime culture as they invented modes of production unique to their special environment. In addition to their sophisticated technical culture, they also attained one of the most complex social organizations of any nonagricultural people in the world. In a division of labor similar to that of the hunting peoples in the interior and among foraging peoples throughout the world, the men did most of the fishing, and the women processed the catch. Women also specialized in the gathering of the abundant shellfish that lived closer to shore. They collected oysters, crabs, sea urchins, mussels, abalone, and clams, which they could gather while remaining close to their children. The maritime life harvested by the women not only provided food, but also supplied more of the raw materials for making tools than did fish gathered by the men. Of particular importance for the native tool kit before the introduction of metal was the wide knife made from the larger mussel shells, and a variety of cutting edges that could be made from other marine shells. The women used their tools to process all of the fish and marine mammals brought in by the men. They cleaned the fish, and dried vast quantities of them for the winter. They sun-dried fish when practical, but

老托福阅读真题 (11)

2002年8月TOEFL试题 Section Three: Reading Comprehension Question 1-9 Often enough the craft worker’s place of employment in ancient Greece was set in rural isolation. Potter, for instance, found it convenient to locate their workshops near their source of clay, regardless of its relation to the center of settlement, At Corinth and Athens, however, two of the best-known potters’ quarters were situated on the cities’ outskirts, and potters and makers of terra-cotta figurines were also established well within the city of Athens itself. The techniques of pottery manufacture had evolved well before the Greek period, but marked stylistic developments occurred in shape and in decoration, for example, in the interplay of black and other glazes with the red surface of the fired pot. Athenian black-figure and red-figure decoration, which emphasized human figures rather than animal images, was adopted between 630 and 530 B.C.；its distinctive color and luster were the result of the skillful adjustments of the kiln’s temperat ure during an extended three-stage period if firing the clayware. Whether it was the potters or the vase-painters who initiated changes in firing is unclear; the functions of making and decorating were usually divided between them, but neither group can have been so specialized the they did not share in the concerns of the other. The broad utility of terra-cotta was such that workers in clay could generally afford to confine themselves to either decorated ware and housewares like cooking pots and storage Jars or building materials like roof tiles and drainpipes, some sixth-and fifth-century B.C. Athenian pottery establishments are known to have concentrated on a limited range of fine ware, but a rural pottery establishment on the island of Thasos produced many types of pottery and roof tiles too, presumably to meet local demand. Molds were used to create particular effects for some products, such as relief-decorated vessels and figurines; for other products such as roof tiles, which were needed in some quantity, they were used to facilitate mass production. There were also a number of poor-quality figurines and painted pots produced in quantity by easy, inexpensive means- as numerous featureless statuettes and unattractive cases testify. 1. The passage mainly discusses ancient Greek pottery and its (A) production techniques (B) similarity to other crafts (C) unusual materials (D) resemblance to earlier pottery 2. The phrase “regardless of” in line 3 is closest in meaning to (A) as a result of (B) no matter what (C) proud of (D) according to 3. It can be inferred from the passage that most pottery establishments in ancient Greece were situated (A) in city centers (B) on the outskirts of cities (C) where clay could be found (D) near other potters’ workshops 4. The word “marked” in line 7 is closest in meaning to

托福阅读真题100篇原文+题目(一)

托福阅读真题100篇原文+题目(一) 托福阅读在备考的过程中，大家可以多找一些真题来进行练习 PASSAGE 1 By the mid-nineteenth century, the term icebox had entered the American language, but icewas still only beginning to affect the diet of ordinary citizens in the United States. The ice tradegrew with the growth of cities. Ice was used in hotels, taverns, and hospitals, and by someforward-looking city dealers in fresh meat, fresh fish, and butter. After the Civil War (1861-1865),as ice was used to refrigerate freight cars, it also came into household use. Even before 1880, halfthe ice sold in New York, Philadelphia, and Baltimore, and one-third of that sold in Boston andChicago, went to families for their own use. This had become possible because a new householdconvenience, the icebox, a precursor of the modern refrigerator, had been invented. Making an efficient icebox was not as easy as we might now suppose. In the early nineteenthcentury, the knowledge of the physics of heat, which was essential to a science of refrigeration,was rudimentary. The commonsense notion that the best icebox was one that prevented the icefrom melting was of course mistaken, for it was the melting of the ice that performed the cooling.Nevertheless, early efforts to economize ice included wrapping the ice in blankets, which kept theice from doing its job. Not until near the end of the nineteenth century did inventors achieve thedelicate balance of insulation and circulation needed for an efficient icebox. But as early as 1803, an ingenious Maryland farmer, Thomas Moore, had been on the righttrack. He owned a farm about twenty miles outside the city of Washington, for which the villageof Georgetown was the market center. When he used an icebox of his own design to transport hisbutter to market, he found that customers would pass up the rapidly melting stuff

历年托福词汇题汇总440题(有答案)

TOEFL词汇题精选440题 001. Most of these leaders were involved in public life as reformers, activists working for The word "representative" is closest in meaning to which of following? (A) typical (B) satisfied (C) supportive (D) distinctive 002. In the United States, Louis Comfort Tiffany (1843-1933) was the most noted exponent of this style, producing a great variety today. The word "prized" is closest in meaning to which of following? (A) valued (B) universal (C) uncommon (D) preserved 003. The Art Nouveau style was a major force in the decorative arts from 1895 until 1915, although its influence continued throughout the mid-1920's. It was eventually to be overtaken by a new school of thought known as Functionalism that had been present since the turn of the century. The word "overtaken" is closest in meaning to which of following? (A) surpassed (B) inclined (C) expressed (D) applied 004. During most of their lives, surge glaciers behave like normal glaciers, traveling perhaps only a couple of inches per day. However, at intervals of 10 to 100 years, these glaciers move forward up to 100 times faster than usual. The word "intervals" is closest in meaning to which of following?(A) records (B) speeds (C) distances (D) periods 005. The increasing water pressure under the glaciers also might be influenced by the climate, volcanic heat, or earthquakes. The word "freeing" is closest in meaning to which of following? (A) pushing (B) releasing (C) strengthening (D) draining 006. A flood of ice would then surge into the turn would release more ice and set in motion a vicious cycle. The word "plunge" is closest in meaning to which of following? (A) drop (B) extend (C) melt (D) drift 007. Group members look to instrumental leaders to "get things done." Expressive leadership, on the other hand, is leadership that emphasizes the collective well-being of a social group's members. The word "collective" is closest in meaning to which of following? (A) necessary (B) typical (C) group (D) particular 008. They offer sympathy when someone experiences difficulties or is subjected to discipline, are quick to lighten a serious

2019托福阅读考试真题(3)

2019托福阅读：模拟试题及答案解析（6）【托福】 Although only 1 person in 20 in the Colonial period lived in a city, the cities had a disproportionate influence on the development of North America. They were at the cutting edge of social change. It was in the cities that the elements that can be associated with modern capitalism first appeared — the use of money and commercial paper in place of barter, open competition in place of social deference and hierarchy, with an attendant rise in social disorder, and the appearance of factories using coat or water power in place of independent craftspeople working with hand tools. "The cities predicted the future," wrote historian Gary. B. Nash, "even though they were but overgrown villages compared to the great urban centers of Europe, the Middle East and China." Except for Boston, whose population stabilized at about 16,000 in 1760, cities grew by exponential leaps through the eighteenth century. In the fifteen years prior to the outbreak of the War for independence in 1775, more than 200,000 immigrants arrived on North American shores. This meant that a population the size of Boston was arriving every year, and most of it flowed into the port cities in the Northeast. Philadelphia's population nearly doubted in those years, reaching about 30,000 in 1774, New York grew at almost the same rate, reaching about 25,000 by 1775. The quality of the hinterland dictated the pace of growth of the cities. The land surrounding Boston had always been poor farm country, and by the mid-eighteenth century it

18年托福阅读真题附答案

The Extinction of the Dinosaurs million years ago) and the Paleocene period (65..C55 million years ago) in part by the types and amounts of rocks and fossils they contain or lack. Before the limit of 65 million years ago，marine 2.strata are rich in calcium carbonate due to accumulations of fossils of microscopic algae deposited on the sea floor. Above the 65-million-year limit，sea-floor sediments contain much less calcium carbonate ，and fossils of several families of mollusks are no longer found. In continental sediments ，dinosaur fossils ，though frequent before 65 million years ago ，are totally absent. By 3.contrast，new families of mammals appear，including large mammals for the first time. Scientists wondered for many years about what could have caused the dinosaurs' rapid disappearance at the end of the Cretaceous period，coming up with a great variety of theories and scenarios. For some, it could have been due to unfavorable genetic changes triggered by a dramatic increaseby a factor of 10，100，1，000 in cosmic-ray particles reaching the Earth after a supernova explosion somewhere in the neighborhood of the solar system. For these high-energy particles to affect life，they would have to get through the protective barrier of the Earth's magnetosphere ，the region of the upper atmosphere controlled by Earth's magnetic field. That could have happened if the cloud of particles from the supernova explosion reached the Earth during a period when the magnetosphere was weakened ， something that may happen when the Earth's magnetic field changes direction. And we know that the magnetic north and south poles of the Earth switch on the average twice every million years. However，this is not the only possible explanation for dinosaur destruction . 4.Other theories have raised the possibility of strong climate changes in the tropics (but they then must be explained) . Certainly，if climate changes，the changed distributions of temperature and rainfall modify the conditions that favor one ecosystem over another. The extinction of a particular family，genus，or species may result from a complicated chain of indirect causes and effects. Over thirty years ago ，scientist Carl Sagan quoted one suggestion that the demise of the dinosaurs resulted from the disappearance of a species of fern plant that was important for dinosaur digestion. Other theories involved a worldwide cold wave following the spread of a layer of cold but not very salty water in the world's oceans ，which floated on the surface because，with its low salinity，the water was less dense. 5.Proponents of another theory that remains under consideration today postulate that the extinction of the dinosaurs corresponds to a period of intense volcanic activity. It's not a question of just one or even of a thousand eruptions comparable to the explosion of Krakatoa in 1883，one of the largest volcanic events in modern times ，but rather of a prolonged period of activity. On the Deccan plateau in India，basalt (volcanic) rocks cover more than 500，000 square kilometers (nearly 200，000 square miles)，and correspond to massive lava outflows occurring precisely at the end of the Cretaceous. This sort of outflow could correspond to volcanic activity similar to the activity that drives sea-floor spreading ，with lava emerging from elongated fractures in the crust rather than from craters. 6.The volcanic convulsion that buried the Deccan plateau in lava must also have changed the composition of the atmosphere and severely affected climate. Initially，there must have been strong sudden cooling resulting from the blocking of sunlight by sulfate aerosol veils in the